1. Field of the Invention
This invention relates to alkali-resistant glass compositions and particularly, though not exclusively, to such compositions which can be formed into glass fibres.
2. Description of the Prior Art
It is known to include a proportion of zirconia (ZrO.sub.2) in glass compositions for enhancing their resistance to alkalis. It is also known that the inclusion of substantial proportions of boron oxide (B.sub.2 O.sub.3) and/or alkali metal oxides (R.sub.2 O) has a deleterious effect on the alkali resistance of the glass, although both these constituents have other favourable properties which have hitherto been considered to make their inclusion desirable. For example, both B.sub.2 O.sub.3 and R.sub.2 O act as fluxes to aid melting and thus help to overcome the tendency of ZrO.sub.2 to make melting difficult, and they can also improve the characteristics of the glass for drawing glass fibres.